The Instrument - the SOHO home page

Contents
Objectives
• Morphology of the Emission Line Corona
• Triggering and Initial Evolution of Dynamical Events
What do we need?
• Coronagraphy
- lens vs mirror coronagraphs
- internally vs externally occulted
What is MICA?
• MICA as an internally occulted mirror coronagraph
- Instrument Layout
- Technical specifications Filters background
Camera background
How does MICA work?
• Observation technique
• Auxiliary Devices
Narrow band imagery

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Set 30, 1998 - Seq
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The K-corona K
(kontinuerliches(
Spektrum)
• White light from the photosphere scattered on free electrons in the partly ionized corona
• Absence of Fraunhofer absorption lines (high electron temperature: Doppler-smear-out)
• Intensity proportional to electron density (summed up along line of sight)
• Strongly polarized, parallel to solar limb
The F-coronaF
The solar corona: : Four different components
• White light from the photosphere, scattered on dust particles
• Continuum spectrum like in the photospeheric one, including Fraunhofer lines
• Very low degree of polarization
• Also known as Zodiacal light
Components Solar Corona I
The T-corona T
(thermal corona)
• Thermal radiation of heated dust particles
• Continuous infrared spectrum, according to temperature of dust particles
• Barely visible
……...

The E-corona E
(emission line corona)
• Line emission from various atoms and ions in the corona
• Strongest line in visible spectral range: 530.3 nm of FeXIV ions (green line)
• Many lines in UV and EUV spectral ranges, e.g., FeXII (19.5 nm), FeXV (28.4 nm)
• Strong radial gradients
• Many forbidden lines, therefore various polarization states
The high ionization state of emitting species reveals
the high temperature of the emission corona.
Components Solar Corona II
Knowledge of the observational characteristics of the different corona components
Understanding of the physical mechanisms responsible of the emission
Separation of the components
Polarizers
Isolation of the K-corona.
Isolation emission lines
Narrow band filters
+
Background subtraction techniques

Relative Intensity
E: Emission Line Corona
F: Fraunhofer Corona
K: Continuum (“white light”) Corona.
Relative intensity of the coronal light components and the sky as
function of solar distance (in solar radii). The relative intensity is
normalized to the intensity at the center of the solar disk.
After van der Hulst, 1953.

Lyot refractive internally occulted coronagraph
Lyot
Coronagraph
LF
R
Oc
LS
O2
FP
A1 O1
FL
A1: entrance aperture
O1: objective lens
Oc: internal occulter,
FL: field lens
LS: Lyotstop
LF: Lyot spot
O2: transfer lens
FP: focal plane.
The solid lines show the rays coming from a point in the
inner corona. The dashed line indicates the path of the
diffracted light at the edges of the entrance aperture.
Internal reflections in the objective lens are denoted with
the letter R.
Lyot observed that each optical element or edge illuminated
by solar radiation gave a contribution to the stray-light level
of the coronagraph.

Refractive externally occulted coronagraph
(Sky tester)
Externally
Occulted
Coronagraph
• Diffracted sunlight from the edge of the external occulting disk
• Residual scattered light in the objective lens itself.
EO
A0
O1
Oc
TL D
LS
E0: external occulter
A0: entrance aperture
O1: objective lens
Oc: internal occulter
LS: Lyot stop
TL: telelens
D: detector (focal plane)

Refractive optics
• Residual scattered light in the objective
lens itself.
• Monochromatic and chromatic aberration
at the position of the internal occulter.
Optimum size and position along the optical
axis of the image of the external occulting
disk is wavelength dependent.
• Aperture limitation due to the problem of
producing large diameter slabs of glass of
requisite quality for the primary objective.
Examples externally occulted
refractive coronagraphs
•LASCO-C2 and C3 onboard SOHO
Brueckner et al., 1995
Constraints
External occulter
Constraints
• Vignetting: Spatial resolution strongly
degraded at the inner edge
of the field of view.
Degradation roughly
proportional to the vignetting
effect in the radial direction.
• Spatial resolution:
Example 1
Diffraction limit
Example 2
Inner fov: set by the distance and size of
the external occulter.
Outer fov: set by the effective aperture of
the objective lens: sinθ = 1.22λ / D
or size of detecting elements.
Dimensional constraints impede to build
sufficiently long externally occulted
coronagraphs to observe the innermost
corona with high spatial resolution.

Refractive optics
Reflective optics
Solutions
External occulter
Solutions
Internal occulter
• Instrumental stray-light can be considerably
reduced by the use of reflective optics
(Newkirk & Bohlin, 1963).
Unlike lenses, mirrors have no problems with
bulk scatter or multiple internal reflections.
• No chromatic aberration at the occulter.
• Large apertures are feasible.
Examples externally occulted
mirror coronagraphs
• Bonnet, 1966
• Kohl et al., 1978
• Smartt, 1979
• No vignetting
Full resolution over the entire field.
Field of view: closer to the limb
• Diffraction limit
Effective aperture of the objective.
Size of detecting element (pixel size).
Internally occulted
mirror coronagraphs
• PICO (Pic Du Midi Coronagraph),
Epple & Schwenn, 1994
• LASCO-C1
Brueckner et al., 1995
• MICA (Mirror Coronagraph for Argentina)
Stenborg et al., 1999

MICA as an internally occulted mirror coronagraph
MICA
Red: photospheric sun light (solar disk)
Blue: diffracted sun light at the edges of A0
Green: scattered sun light + coronal light
instrumental
in the sky
K-corona
E-corona

MICA Technical Data
MICA technical data
Elem. Type Aperture
(in mm)
Curvature
(in mm)
Remarks
A0 Circ. 59 - Entrance
Aperture
M1 Off-axis 90 FL = 750 Primary Mirror
Parabola
M2 Convex ID=7 R = 2422 Occultor
Sphere OD=20
M3 Off-axis 90 FL = 750
Parabola
S Shutter 40 - Mechanical
A1 Annular ID=38.4 - Lyot Stop
Aperture
TL Telelens -
CCD Camera 16 µ/pxl - 1280x1024 pxl
(~3.8 arcsec/pxl)

λ
(nm)
FWHM
(nm)
Max.Trans.
Flatness
The filters in MICA
Fe XIV
(line)
Fe XIV
(cont)
Fe X
(line)
530.3 0.15 + 52 % λ/4/45 mm
526.0 1 56 % λ/4/25 mm
637.4 0.15 57 % λ/4/45 mm
The filters in MICA
Fe X
(cont)
634.0 1 36 % λ/4/25 mm
1.05 MK 1.80 MK
• Model of the continuum at the
wavelength of the on-line filter.
- Far from line center, to avoid
contamination by emission in the
line itself.
- Relatively broad passband to be
sure that no major emission lines
contribute to the measured
continuum flux.
Relative Abundance [%]
100
10
1
Temperature [10 6 °K]
0.63 1.00 1.58 2.51
Fe X
Fe XIV
5.8 6.0 6.2 6.4
log(T)
Log(T)

Principle of an interference filter
Multiple reflections
between two paralell
surfaces
Filter Theory
Practical interference filter used at
normal incidence
Transparent dielectric
θ
• Maxima at (normal incidence θ = 0)
λ ( = )
Transmission
Wavelength
Tmax
=
⎛ λ − λ
1+ ⎜ ⎝ ∆/2
2
2
sin ( θ ) θ →0
θ
• Tilting by θ ==> λm
θ
= λm
1−
⎯⎯ →λ
2
mθ
− λ0
≈
2
n
2n
T
0
0
⎞
⎟
⎠
Glass plates
Metallic films
Reflectivity of the metal
films determine width of
the maxima

Filter Transmission Classical
Solar spectrum
Shifting of the green line filter
passband with temperature and
solar distance
Filter in the parallel beam
FWHM = 0.15 nm
green line profile
Solid line (black): filter transmission at the temp.
where the green line emission close to the limb is
observed to be maximum, i.e, T 0 .
Dashed line (brown): filter passband at the edge
of the field of view, at the same temperature T 0 .
Dashed-dotted-dotted line: filter passband close
to the limb at T 0 +3° (red) and T 0 -3° (blue).
Temperature coefficient: K T = 0.0168 nm/°C.

Filter Transmission Homocentric
Shifting of the green line filter
passband with solar distance
Filter in homocentric beam
FWHM = 0.15 nm
For comparison, the dashed line
(black) shows the passband of a filter
located in the parallel beam.
Dotted line (black): filter transmission at normal
incidence.
Solid line (red): filter transmission across the field
of view.

S
E
W
N
On-line
Off-line
N
Orientierung
W
N
E
S
E
W
Equator
S

MICA Block Diagram

The diffraction limit is the minimum angular separation that two
different sources must have in order to be resolved by the
telescope.
Diffraction Limit Definition

Chromatic aberration occurs due to the variation of refractive
index with wavelength for a lens material. This wavelength
dependence results in slightly different focal lengths for different
wavelengths of light. Therefore, the lens produces a coloured
blurring rather than a true color, sharply focussed image.
Chromatic Aberration Definition

Vignetting effect
C2 Spatial Resolution
Brueckner et al., 1995. (Sol. Phys., 162, 357)

Vignetting effect
C3 Spatial Resolution
Brueckner et al., 1995. (Sol. Phys., 162, 357)

Construction of a typical filter
Construction of a typical filter
• Glass plates: no role. Mechanical support for the filter.
• Surface of one plate coated with:
- a thin evaporated film of high reflectivity,
- a thin layer of a transparent dielectric, and
- another metal film.
•The other plate is added for protection.